In recent years, lithium secondary batteries, nickel hydrogen batteries and other secondary batteries (storage cells) have been growing in importance as on-board electric power sources for vehicles and electric power sources for personal computers and handheld devices. In particular, lithium secondary batteries, which are lightweight and can achieve a high energy density, are advantageously used as high-power on-board electric power sources. In a typical construction, this type of lithium secondary battery has a positive electrode, a negative electrode, and a porous separator between the positive electrode and the negative electrode. The separator prevents short circuits associated with contact between the positive electrode and the negative electrode. In addition, pores within the separator are impregnated with an electrolyte, thereby forming ion-conducting paths between both electrodes.
The separators used up until now have been porous resin sheets made of for example, polyethylene (PE) or polypropylene (PP). Because such separators are porous, heat shrinkage arises at elevated temperatures. This effect is used to actuate a shutdown function. However, if the degree of thermal shrinkage is large, localized shorting due to film breakage or the like may arise, and shorting may spread further from this point. Hence, to prevent heat shrinkage of the separator, the formation of a porous heat-resistant layer on the separator surface has been disclosed (see, for example, Patent Literature 1). The formation of a porous heat resistant layer at the surface of the positive electrode or the negative electrode in order to keep the positive electrode and the negative electrode from coming into direct contact when the separator undergoes heat shrinkage has also been investigated.